Vehicle suspension systems are provided to control the relative motion between the vehicle chassis and its wheels. Such systems try to isolate the body of the vehicle from vibrations and bumps resulting from road irregularities and to resist inertial roll and pitch motions of the body which result from braking, accelerating and cornering. There is a tension between achieving both of these objectives because vibration isolation requires a relatively low opposing force (i.e, the damping should be relatively "soft ") and inertial resistance requires a relatively high opposing force (i.e, relatively "hard " damping).
Active and semi-active systems have been developed to overcome this limitation. They employ sensors to measure various vehicle and/or wheel conditions and a microprocessor to interpret the sensor data in order to provide one or more control signals which continually adjust each shock absorber on the vehicle in response to the operating conditions at any point in time.
Active systems include a hydraulic pump controlled by the control signals. The pump powers an apparatus which actually moves the wheels up and down independently of the motion inputs from the vehicle body and the wheels. While such active systems can provide excellent performance, they are costly and heavy and use a significant amount of the energy produced by the vehicle engine.
Semi-active systems have been devised to overcome these problems. They provide some controlled variability to the typical type of passive shock absorber which employs a piston and cylinder assembly having chambers of variable volume which are connected by an orifice through which hydraulic fluid is displaced. In such a device, the cross sectional area of this orifice determines the force exerted by the shock absorber in resistance to the pressure force created in one of the chambers as a result of relative motion between the vehicle body and the wheels.
In a passive system, the size of the inter-chamber orifice is fixed. Thus the system is set at a compromise point on the hardness/softness spectrum so that it is less than ideal for achieving both objectives of vibration isolation and inertial resistance. Further, the shock absorber's resistive force (which is proportional to the pressure drop across the orifice) is proportional to the flow rate through the orifice, which varies positively with the input pressure--i.e., the shock absorber hardens as input pressure increases and softens as it decreases, which may be contrary to the desired result.
Like an active system, a semi-active system employs control signals to control the performance of the suspension system. It does not, however, employ a pump. Rather, in a semi-active system the control signals constantly change the area of the inter-chamber orifice through which the hydraulic fluid in the piston/cylinder assembly flows in response to wheel and body movements. These changes adjust the shock absorber's point on the hardness/softness spectrum to that which is the most appropriate for the conditions at each moment.
Semi-active shock absorbers generally employ an electrically powered actuator (such as a solenoid) to vary the area of the orifice in response to the control signals. If the actuator had to control an orifice in the main flow path between the chambers, it would have to be large and strong enough to handle very substantial forces which would result from a high flow rate. As a result, it would be costly, bulky and heavy, and it would consume significant amounts of electrical power. To reduce this problem, it is known to have the actuator vary an orifice in a pilot passage which controls the main valve. See U.S. Pat. No. 4,902,034, Feb. 20, 1990, Maguran et al. In such an apparatus, the resistive force can be made substantially independent of flow rate, once a small minimum flow rate has been exceeded. As a result, the resistive force depends almost entirely on the level of electrical current input to the actuator. There is, however, a need for a simpler, more compact and less expensive semi-active system employing a pilot circuit.
In most driving situations, the inter-chamber orifice of a semi-active system will be opening and closing only slightly. Nonetheless, it must be large and strong enough to handle the high flow rates which may be needed in other situations. In those typical driving situations, this size tends to slow the moment-to-moment response and to consume more power than would otherwise be necessary. Therefore, there is a need for a system which provides faster response and uses less power in the routine driving situation.
It is often desirable that the compression chamber (i.e., the chamber between the piston and the vehicle body) and the rebound chamber (i.e., the chamber between the piston and the wheel) be at different points of the hardness/softness spectrum. For example, relatively hard damping in the compression chamber and relatively soft damping in the rebound chamber would be desirable on the right shock absorbers when the car swerved left. The hard compression chamber would prevent the car's right side from pitching down sharply and the soft rebound chamber would allow the car to quickly recover from any pitch that did occur. Conversely, in a sharp left turn, the left shock absorber should be in hard rebound and soft compression in order to cooperate to achieve the same result. Thus, in many driving situations, it is desirable to control both the compression and the rebound chamber such that to the extent that one is hard the other is correspondingly soft, and vice versa. There is a need for a system which can accomplish this result simply and economically.